The invention relates to a gas chromatograph, more particularly, a gas chromatograph wherein a flow rate of a carrier gas to be supplied to a sample introducing portion is measured and controlled by a differential pressure.
In a sample analysis, in case a sample is analyzed by a gas chromatograph, the sample is introduced into a sample introducing portion and then introduced into an analyzing column together with a carrier gas. At that time, the carrier gas to be introduced into the sample introducing portion is required to be precisely controlled in order to measure an accurate quantity and characteristics in the analysis. In a conventional technique, a flow rate of the carrier gas has been measured and controlled by a differential pressure sensor.
FIG. 1 shows a structure of a conventional gas chromatograph. Reference numeral 1 represents a bomb for supplying a carrier gas; 2 represents a pressure regulator; 3 represents a resistance tube; 4 represents a differential pressure sensor; 5 represents a control valve; 6 represents a sample introducing portion; 7 represents an analysis column; 8 represents a detecting portion; and 9 represents a control portion. The differential pressure sensor 4 measures a differential pressure on both sides of the resistance tube 3. Among a flow rate of the carrier gas, a pressure on an upstream side of the resistance tube, and a differential pressure measured by the differential pressure sensor 4, the following equation is held: EQU Q=K.times.P1.times..DELTA.P (1),
wherein Q denotes the flow rate of the carrier gas; K denotes a coefficient determined by the resistance tube 3; P1 denotes a pressure on an upstream side of the resistance tube 3; and .DELTA.P denotes a differential pressure measured by the differential pressure sensor 4.
An analyst inputs a desired flow rate, i.e. Q-SET, to the controlling portion. The controlling portion 9 memorizes Equation (1) therein. P1 in Equation (1) is a fixed value set by the pressure regulator 2 as shown in FIG. 1. In the controlling portion 9, after a differential pressure .DELTA.P-SET corresponding to the flow rate Q-SET which the analyst desires is obtained from Equation (1), a signal is supplied to the control valve 5 so that a differential pressure .DELTA.P measured by the differential pressure sensor 4 becomes .DELTA.P-SET, to thereby control the flow rate of the valve. The carrier gas, the flow rate of which has been controlled by the controlling valve 5, carries a sample introduced into the sample introducing portion 6 to the analyzing column 7 and then to the detecting portion 8. After the sample is separated by the analyzing column 7, the sample is subjected to analysis characteristics and quantity.
A pressure P-IN supplied from the bomb 1 varies depending on a remaining quantity, temperature and the like of the carrier gas in the bomb 1. In case the pressure regulator 2 is not provided, P-IN=P1. In case the pressure regulator 2 is not provided and the P-IN has changed, P1 which should be a fixed value in Equation (1) changes, so that Equation (1) does not exhibit a correct relationship between an actual flow rate of the carrier gas and the differential pressure. Therefore, the pressure regulator 2 should be provided on the upstream side of the resistance tube 3, which must be an expensive pressure regulator to keep the pressure P1 on the upstream side of the resistance tube 3 uniform even when the P-IN has been changed.
Further, in order that the gas flows out of the bomb, the pressure P-IN should be larger than the pressure P1, and it is required to supply the carrier gas with a pressure higher than that of the fixed pressure of the pressure regulator.
In view of the above, the present invention has been made, and an object of the invention is to provide a gas chromatograph wherein an expensive pressure regulator is not required, and moreover, a supply pressure of a carrier gas can be selected as desired.
Further objects and advantages of the invention will be apparent from the following description of the invention.